An image forming apparatus of a tandem system in which photosensitive drums corresponding to respective colors are arranged in a direction orthogonal to a rotating shaft and developer (e.g. toner) images formed on surfaces of the respective photosensitive drums are serially transferred to and overlaid on a recording medium or an intermediate transfer member is known in the art. Since drum gears are fixed to ends of the respective photosensitive drums for driving the respective photosensitive drums and the drum gears are generally formed of resin, they may easily shift, which is undesirable, due to a manufacturing error in dimension. For example, when the drum gear has been shifted to an off-centered position, uneven rotation of the photosensitive drum results and fluctuations in rotational speed of the respective photosensitive drums can cause a color drift in the overlaid developer images.
As a consequence, in order to suppress the color drift based on uneven rotation occurring on the photosensitive drums, gears formed by an identical molding die are fixed to the ends of the respective photosensitive drums as described in JP-A-9-179372, and the respective drum gears are provided with marks so that the rotational speeds of the respective photosensitive drums match when the developer images are transferred to the identical position on the recording medium or the intermediate transfer member.
Photosensitive drums are detachably attached to a main body of the image forming apparatus to allow them to be replaced when they deteriorate or wear out over time.
However, in such a configuration in which the photosensitive drums can be attached to and detached from the main body, even though the drum gear is aligned with a predetermined rotational position during manufacturing, there is a risk that the relative rotational position of the drum gears can be displaced when a user moves the gears when handling a paper jam or the like. Therefore, the rotational speeds of the respective photosensitive drums may vary at the time of transfer, which can cause color drift to occur in the formed images.